Rearrangement of Multiferroic BiFeO3 Nanodots Smaller than 15 nm Using Atomic Force Microscopy

In this study, we demonstrate an atomic force microscopy process for manipulating multiferroic BiFeO₃ nanodots smaller than 15 nm to desired positions on a Nb‐doped SrTiO₃ substrate. For formation of the BiFeO₃ nanodot array, nanocrystal movement was achieved using a +1.2 V biased conducting atomic force microscopy (CAFM) followed by nanocrystal attachment to the tip. Using this method, high‐density BiFeO₃ nanodot arrays with a density greater than 0.5 Tb/in.² can be achieved. Perfectly flipped ferroelectric polarization with an external electric field was observed for each BiFeO₃ nanodot, whose ferroelectric properties were confirmed using piezoelectric force microscopy.     

Electric‐Field‐Induced Domain Switching and Domain Texture Relaxations in Bulk Bismuth Ferrite

Bismuth ferrite, BiFeO₃, is an important multiferroic material that has attracted remarkable attention for potential applications in functional devices. While thin films of BiFeO₃ are attractive for applications in nanoelectronics, bulk polycrystalline BiFeO₃ has great potential as a lead‐free and/or high‐temperature actuator material. However, the actuation mechanisms in bulk BiFeO₃ are still to be resolved. Here we report the microscopic origin of electric‐field‐induced strain in bulk BiFeO₃ ceramic by means of in situ high‐energy X‐ray diffraction. Quantification of intrinsic lattice strain and extrinsic domain switching strain from diffraction data showed that the strain response in rhombohedral bulk BiFeO₃ is primarily due to non‐180° ferroelectric domain switching, with no observable change in the phase symmetry, up to the maximum field used in the study. The origin of strain thus differs from the strain mechanism previously shown in thin film BiFeO₃, which gives a similar strain/field ratio as rhombohedral bulk BiFeO₃. A strong post‐poling relaxation of switched non‐180° ferroelectric domains has been observed and hypothesized to be due to intergranular residual stresses with a possible contribution from the conductive nature of the domain walls in BiFeO₃ ceramics.     

Magnetoelectric response from the enhanced ferromagnetism and flexoelectric response in reduced BiFeO3–based ceramics

The magnetoelectric effect of conventional magnetoelectric materials originates from the coupling between ferroelectricity and ferromagnetism. In this work, we demonstrate that a magnetoelectric response can also be achieved through the coupling between flexoelectricity and ferromagnetism. We show that the ferromagnetism and apparent flexoelectric response of BiFeO₃-based ceramics are greatly enhanced after an asymmetrical chemical reduction is applied to the materials. The reduction also induces an inhomogeneous magnetic property in the materials, resulting in a bending deformation and flexoelectric response under a magnetic field. A magnetoelectric response, evidenced by an abrupt change in magnetic-field-induced electrical signals near the resonance frequency of the bending deformation, is obtained in the reduced BiFeO₃-based ceramics. The present work provides an approach for designing magnetoelectric materials by exploiting the flexoelectric effect of materials.     

In situ study of electric-field-induced ferroelectric and antiferromagnetic domain switching in polycrystalline BiFeO3

Antiferromagnetic domain switching induced by ferroelectric polarization switching has previously been observed in situ in both multiferroic BiFeO₃ single crystals and thin films. Despite a number of reports on macroscopic magnetoelectric measurements on polycrystalline BiFeO₃, direct in situ observation of electric-field-induced antiferromagnetic domain switching in this material has not been addressed due to the lack of high-quality samples capable of electrical poling. Here, the electric field control of antiferromagnetic domain texture is identified in polycrystalline BiFeO₃ using in situ neutron diffraction, showing the resultant magnetic domain reorientation induced by an electric field. An antiferromagnetic domain reorientation to a value of 2.2-2.5 multiples of a random distribution (MRD) is found to be induced by an electric field that provides a non-180° ferroelectric-ferroelastic domain texture of 2.2-2.5 MRD along the field direction. The current results show well-controlled coupling of multiferroic domain texturing in single-phase polycrystalline BiFeO₃.     

Focus on the ferroelectric polarization behavior of four-layered Aurivillius multiferroic thin film

The well-saturated ferroelectric hysteresis loops with double remnant polarization up to 50?μC/cm² were obtained in four layered Aurivillius-type multiferroic Bi₅FeTi₃O₁₅ thin film. Pulsed positive-up negative-down polarization measurements demonstrate the intrinsic ferroelectric polarization, which present optimal rectangularity and polarization value. The hysteresis loops measurements with larger frequency range of 0.2–100?kHz indicate stable and ultra-fast switching speed of ferroelectric domains. Persistent retention properties were observed, and they are also independent of the applied electric field. In fatigue test an increased dielectric constant is observed along with the suppression of switchable polarization. Both of them can be restored partly to their original values via the stimulating of high electric field. The block domain switching due to the oxygen vacancies aggregated on domain walls are discussed for those characteristics. It is providing important contributions of domain wall pinning in the polarization degradation of Aurivillius-type ferroelectric films with four layers.     

Coercive field modified via partial ion substitution,mechanical load and charge injection in (Ba,Ta, Cr) doped BiFeO3 films

Changes in the coercive field of ferroelectric materials translate into variations in the operating power consumption of ferroelectric devices. Here, we have used partial ion substitution to reduce the coercive field in BiFeO₃ films deposited by the pulsed laser technique. We have also used mechanical loads without interruption during switching events at the nanoscale to follow the changes in the coercive field. Furthermore, we have analyzed the dependence of the coercive field concerning the injected charges during the writing process. The partial ion substitution of Ba, Ta, and Cr in the BiFeO₃ structure causes a reduction of ∼50% in the coercive field due to a weakening in the A_sites-O bonds. The mechanical load experiments show that linearly increasing mechanical loading from ∼50 nN to ∼600 nN tends to increase the signal amplitude of the piezoresponse and move the coercive field locations. The linear unloading reduces the amplitude and returns the coercive field to values close to before the force experiment, pointing out a reversible process. Such behavior can be explained by considering that the piezoelectric response is supported and enhanced by the electric field created by the flexoelectric effect during the mechanical loading process due to the small tip radius. Finally, the application of negative (positive) voltage during the writing process caused the loops to shift to the left (right), changing the locations of the coercive field.     

Large domain-wall currents in epitaxial Au/BiFeO3/SrRuO3 thin-film capacitors with modulated oxygen vacancy and wall densities

Large domain wall (DW) conductivity in an insulating ferroelectric plays an important role in the future nanosensors and nonvolatile memories. However, the wall current was usually too small to drive high-speed memory circuits and other agile nanodevices requiring high output-powers. Here, a large domain-wall current of 67.8 μA in a high on/off ratio of ~4460 was observed in an epitaxial Au/BiFeO₃/SrRuO₃ thin-film capacitor with the minimized oxygen vacancy concentration. The studies from read current-write voltage hysteresis loops and piezo-response force microscope images consistently showed remaining of partially unswitched domains after application of an opposite poling voltage that increased domain wall density and wall current greatly. A theoretical model was proposed to explain the large wall current. According to this model, the domain reversal occurs with the appearance of head-to-head and tail-to-tail 180° domain walls (DWs), resulting in the formation of highly conductive wall paths. As the applied voltage increased, the domain-wall number increased to enhance the on-state current, in agreement with the measurements of current-voltage curves. This work paves a way to modulate DW currents within epitaxial Au/BiFeO₃/SrRuO₃ thin-film capacitors through the optimization of both oxygen vacancy and domain wall densities to achieve large output powers of modern domain-wall nanodevices.     

Domain reversal and current transport property in BiFeO3 films

The electrical properties and domain reversal in BiFeO₃ ferroelectric films were studied using sandwiched heterostructures and piezoresponse force microscopy. A robust polarization state was observed, combined with a switchable domain pattern and a remanent polarization of approximately 100 μC cm^?2. In addition, domain reversal was explored using scanning probe microscopy. The results show that dipoles could be reversed along the direction of the electric field under a negative tip bias, leading to carrier gathering near the domain walls. The enhanced conductivity near the domain walls was owing to the discontinuous polarization boundary conditions. In addition, typical diode-like current transport properties are sensitive to various temperature conditions, which is attributed to the Schottky barriers at the contact interface. These findings extend the current understanding of domain texture reversal in ferroelectric films and shed light on their potential applications for future ferroelectric random-access memory operations over a wide temperature range.     

Ferroelectric photovoltaic and flexo-photovoltaic effects in (1 − x)(Bi0.5Na0.5)TiO3-xBiFeO3 systems under visible light

The anomalous photovoltaic (APV) effect has witnessed great progress from classical ferroelectric photovoltaics to flexo-photovoltaics. Both call for an extension of the spectral response range. Here, we present a comprehensive study on the ferroelectric, photoelectric, and photovoltaic properties of pure (Bi_0.5Na_0.5)TiO₃ (BNT) and 0.3(Bi_0.5Na_0.5)TiO₃–0.7BiFeO₃ (0.3BNT-0.7BFO) ceramics. The data show that pure BNT with typical ferroelectricity exhibits intriguing photovoltaic effect even when illuminated by 550 nm visible light, while the 0.3BNT-0.7BFO solid solution with enhanced visible light absorption shows no ferroelectric photovoltaic due to the negligible ferroelectricity but striking strain-induced flexo-photovoltaic effect. Transmission electron microscopy analysis reveals distinctions in the domain structures and local strain states in pure BNT and 0.3BNT-0.7BFO ceramics, which may account for their differences in photovoltaic behavior. These findings not only deepen the understanding of photovoltaic mechanisms induced by ferroelectric polarization and flexoelectric effect but also highlight a possible candidate for multifunctional photoelectric applications.     

BiFeO3 Ceramics: Processing,Electrical, and Electromechanical Properties

Bismuth ferrite (BiFeO₃), a perovskite material, rich in properties and with wide functionality, has had a marked impact on the field of multiferroics, as evidenced by the hundreds of articles published annually over the past 10 years. Studies from the very early stages and particularly those on polycrystalline BiFeO₃ ceramics have been faced with difficulties in the preparation of the perovskite free of secondary phases. In this review, we begin by summarizing the major processing issues and clarifying the thermodynamic and kinetic origins of the formation and stabilization of the frequently observed secondary, nonperovskite phases, such as Bi₂₅FeO₃₉ and Bi₂Fe₄O₉. The second part then focuses on the electrical and electromechanical properties of BiFeO₃, including the electrical conductivity, dielectric permittivity, high‐field polarization, and strain response, as well as the weak‐field piezoelectric properties. We attempt to establish a link between these properties and address, in particular, the macroscopic response of the ceramics under an external field in terms of the dynamic interaction between the pinning centers (e.g., charged defects) and the ferroelectric/ferroelastic domain walls.     

Polar domain structural evolution under electric field and temperature in the (Bi0.5Na0.5)TiO3-0.06BaTiO3 piezoceramics

Lead-free bismuth sodium titanate and related compounds are of great interest as promising candidates for piezoelectric applications. However, the full understanding of this family of materials is still a challenge partly because of their structural complexity and different behaviors with or without the application of an external electric field. Here, piezoresponse force microscopy is used to gain insight into the mesoscopic-scale domain structure of the morphotropic phase boundary (MPB) composition of (1-x)Bi_0.5Na_0.5TiO₃-xBaTiO₃ solid solution at x = 0.06 (abbreviated as BNT-6BT). The evolution of the domains with the changes of the electric field and temperature has been thoroughly examined in conjunction with the crystal structure analysis and dielectric studies. It is found that ferroelectric domains with size of hundreds of nanometers are embedded in a relaxor state without visible domains on a mesoscopic scale, which are considered to contribute to the tetragonal and cubic phases in the material, respectively. Temperature-independent domain configuration is observed in the unpoled sample from room temperature to 200°C. While, temperature-dependent domain configuration is observed in the poled sample. The homogenously poled state breaks into the mixed domain configuration containing polydomain structure and invisible state around the so-called depoling temperature. The structural changes on different length scales are also discussed. This work provides an in-depth understanding of the structural and domain changes under an electric field and the temperature-dependent domain evolution in both unpoled and poled states in the BNT-BT solid solution of the MPB composition.     

Frequency dependence of the relaxor-to-ferroelectric transition under applied electrical and mechanical fields

Relaxor ferroelectrics are a unique material class with underlying microscopic processes that are yet to be fully understood. In particular, the electrical and mechanical field-modulated transition from the relaxor state to long-range ferroelectric order has been found to be the origin of the large unipolar strain response in some lead-free ferroelectric systems. Importantly, the dynamics of this transition are different than the domain wall nucleation and growth found in normal ferroelectrics, significantly changing the frequency response. In this study, the electromechanical behavior of a (Na_1/2Bi_1/2)TiO₃-based lead-free relaxor ferroelectric is shown under applied external mechanical and electric field over several orders of magnitude in loading rate. Depending on the loading history, it is possible to directly investigate both the relaxor-to-ferroelectric transition as well as domain wall motion in the same sample. These data demonstrate the variations in frequency response between these nonlinear hysteretic processes.     

Room-temperature magnetic control of ferroelectric polarization and enhanced ferromagnetic properties in 0.8BiFeO3-BaTiO3 ceramic

A multiferroic 0.8BiFeO₃ ??0.2BaTiO₃ (0.8BF-BaT) ceramic was prepared by solid-state reaction to study the magnetic control of its ferroelectric polarization and ferromagnetic properties. The influence of the magnetic field H on the ferroelectric polarization of 0.8BF-BaT ceramic was investigated. The maximum displacement current density Jm and the remanent polarization Pr of 0.8BF-BaT increased by 35% and 20% respectively after it being magnetized, indicating that the polarization was suppressed in the magnetic domain walls. Under a magnetic field of H?=?0.7?T, Jm and Pr decreased by 6.1% and 6.4% respectively, revealing coupling between polarization P and magnetic field H. The magnetic properties of BiFeO₃ were improved by introducing of BaTiO₃ and the magnetic Néel temperature T_N increased by 70?K because of structural effects.     

Temperature Gradient Introduced Ferroelectric Self‐Poling in BiFeO3 Ceramics

The as‐prepared BiFeO₃ ceramic shows a piezoelectric d₃₃ coefficient of ?14 pC/N, that is, an obvious ferroelectric self‐poling phenomenon. The temperature gradient between the two surfaces of BiFeO₃ ceramic was intentionally enlarged when BiFeO₃ was prepared with a rapid liquid sintering method. This temperature gradient and the corresponding thermal strain can introduce defect dipoles through separating bismuth vacancies from oxygen vacancies. A mass of these dipoles introduce a macroscopic internal electric field (E_in) which downward poles BiFeO₃ ceramic during its cooling down process. As expected, an E_in of >10 kV/cm is confirmed by the asymmetrical polarization/strain versus electric field curves.     

Electric field-induced two-step phase transformation and its contribution to the electromechanical strain in lead-free relaxor-based ceramics

Sodium bismuth titanate-based solid solutions are important lead-free piezoelectrics with potential applications. Large electromechanical strain is of particular interest, which can be realized via the structural change in the relaxor states under an electric field. In this work, a polycrystalline ceramic 0.85Na_1/2Bi_1/2TiO₃–0.15BaZr_0.2Ti_0.8O₃, in which a long-range ordered ferroelectric phase and a relaxor state coexist, has been investigated to unveil the origin of its electromechanical strain using various experimental techniques. An ergodic-relaxor to nonergodic-relaxor transition is first observed under a relatively weak electric field, and a more stable long-range ferroelectric phase is induced under a larger electric field. This two-step phase transformation is accompanied with the process of local polarization freezing as well as ferroelectric domain growth. The domain formation during the reversible phase transition is found to be the main contribution to the macroscopic strain. Our investigation provides an in-depth understanding of the origin of reversible electromechanical strain in the NBT-based relaxor-ferroelectric system.     

Ferroelectromagnetic characteristic of Na-doped 0.75BiFeO3–0.25BaTiO3 multiferroic ceramics

BiFeO₃-based materials are expected to have both ferroelectricity and ferromagnetism simultaneously. In this study, effects of Na-doping (0.5, 1.0, 3.0, and 5.0 mol%) on ferromagnetic and ferroelectric properties of 0.75BiFeO₃–0.25BaTiO₃ ceramics which have been fabricated by the solid state reaction technique are studied. The effects of Na-doped 0.75BiFeO₃–0.25BaTiO₃ ceramics on the crystal structure, and magnetic and electrical properties were investigated and discussed. Rhombohedrally distorted 0.75BiFeO₃–0.25BaTiO₃ showed weak ferromagnetic and ferroelectric properties. In addition, ferroelectric and ferromagnetic properties of 0.75BiFeO₃–0.25BaTiO₃ have been controlled by Na doping, and the maximum values of magnetization and polarization were observed at 5.0 mol%.     

Fabrication of epitaxial ferroelectric BiFeO3 nanoring structures by a two-step nano-patterning method

A novel two-step nano-patterning method is proposed to fabricate epitaxial ferroelectric BiFeO₃ (BFO) nanoring array, which maintains well-epitaxial structure and possesses strong ferroelectricity demonstrated by X-ray diffraction (XRD) and piezoresponse force microscopy (PFM). The ferroelectric polarizations were examined by PFM, revealing the reversible switching behavior under an electric field. This novel method could also be extended to other oxide material systems. The fabrication of high quality ferroelectric nanoring structure provides the possibility to explore novel functionalities (e.g., ferroelectric vortices) and offers application potentials for the high-density non-volatile memory devices.     

The evolution of structure,properties and polar domains in rare earth and PbTiO3 co-substituted BiFeO3 ferroelectric ceramics

BiFeO₃ (BFO) based ferroelectric solid solutions attract long-lasting research interests due to their multi-functionalities including electric/multiferroic/energy-storage properties. However, achievement of large ferroelectric polarization is still highly challenging in BFO based bulk ceramics due to large leakage. In this work, the structure and electrical properties of rare earth Nd- and PbTiO₃ co-modified BFO ceramics have been explored. Based on high temperature in-situ X-ray diffraction and dielectric measurements, a preliminary ferroelectric phase diagram is established, depicting the morphotropic phase boundaries (MPB) and a critical temperature that cannot be correlated to any macroscopic phase transition. The effects of rare earth substitution on structure evolution have been investigated by comparing the results in this work and literature. The accomplishment of ferroelectric switching with giant ferroelectric polarization above 65 μC/cm² is successfully achieved without resorting to quenching treatment. The MPB compositions demonstrate the maximum piezoelectric coefficients and the lowest coercive field, suggesting the “softening” effects. The domain evolutions suggest two coexisting phases in MPB composition distribute separately in different grains.     

Electrical fatigue behavior of Ba0.85Ca0.15Zr0.1Ti0.9O3 ceramics under different oxygen concentrations

Fatigue degradation is a significant problem in piezo/ferroelectric materials and their commercial applications. The major causes of electrical fatigue degradation are a domain pinning effect and physical damage such as microcracking. This work reports the fatigue behavior of barium calcium zirconate titanate (Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃) under regular and low oxygen concentration silicone oil. Impedence analyzer and LCR meter are employed to analyzer the dielectric properties and it also revealed the relationship between activation energy and oxygen vacancy. X-ray diffraction, synchrotron X-ray absorption spectroscopy, and scanning electron microscope techniques were employed to study the local structural changes, defect development, physical damage and microcracking in the ceramics. FEFF-8.4 simulations were used to determine the oxygen vacancy creation. The study reveals the relationship of oxygen vacancy creation, domain wall pinning, microcracking and the fatigue behavior of the ferroelectric ceramic. The work investigated the dielectric and ferroelectric properties of BCZT ceramics intermes of applied electric field.     

Synthesis and properties of multiferroic 0.7BiFeO3−0.3BaTiO3 thin films by Mn doping

Multiferroic BiFeO₃?BaTiO₃ thin films that simultaneously exhibit ferroelectricity and ferromagnetism at room temperature were prepared by chemical solution deposition. Perovskite single-phase 0.7BiFeO₃?0.3BaTiO₃ thin films were successfully fabricated in the temperature range 600–700 °C on Pt/TiO_x/SiO₂/Si substrates. As the crystallization temperature was increased, grain growth proceeded, resulting in higher crystallinity at 700 °C. Although the 0.7BiFeO₃?0.3BaTiO₃ thin films exhibited poor polarization (P)?electric field (E) hysteresis loops owing to their low insulating resistance. The leakage current at high applied fields was effectively reduced by Mn doping at the Fe site of the 0.7BiFeO₃?0.3BaTiO₃ thin films, leading to improved ferroelectric properties. The 5 mol% Mn-doped 0.7BiFeO₃?0.3BaTiO₃ thin films simultaneously exhibited ferroelectric polarization and ferromagnetic magnetization hysteresis loops at room temperature.